Differential scanning calorimetry studies on structural phase transitions and molecular motions in (CH3NH3)(2)MBr4 and [(CH3)(2)NH2](2)MBr4 (M = Zn, Cd, and Hg)

摘要

Structural phase transitions and molecular motions in (CH3NH3)(2)MBr4 and in [(CH3)(2)NH2](2)MBr4 (M = Zn, Cd and Hg) were investigated by differential scanning calorimetry (DSC) between about 130 K and the melting points. All of the compounds showed at least one structural phase transition over the temperature region investigated. The values of the transition entropies obtained suggest that these transitions are of the order-disorder type. N-H ... Br hydrogen bonds were found to play an important role in the stability of P2(1)/c room-temperature phases of the title compounds. In the highest-temperature solid phases of all title compounds, orientational orders of cations seem to be completely lost. [References: 26]